<span>The answer is a certain required chemicals must be actively
transported. Blood brain barrier is a
selective membrane that separates blood circulating from the brain and the
fluid of extracellular in the central nervous system. This is also called a
filtering mechanism, the capillaries that carry blood to the brain and spinal
cord tissue blocking the path way of the certain substance.</span>
<span>The mountains were once seas. The land was forced up by tectonic motion when the mountains were formed. The Sahara also used to be a sea and has a valley full of the fossilized remains of an extinct species of whale. It is generally agreed upon that two plates colliding caused the uplift that created the mountainous zone. Same for the Himalayas.</span>
Waves erode a beach by pressing continually against them, right? I have heard of this happening many times. What the people do is build a small trench around the outside of the beach so the waves filter down into it, then back out instead of washing up against the sand. The other thing they do is wait a couple years for the beach to go down, then they put in new sand during the winter.
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A is the answer ur welcome for helping u
I believe a valley or mountain, but I am not 100 percent sure.
